1. Field of the Invention
The present invention relates, generally, to methods and networks for linearizing an FM/CW radar and, more especially, to such methods and networks which will permit an FM/CW radar to achieve target discrimination. The present invention is particularly adapted for use in association with a missile, permitting target discrimination from background clutter and/or false targets. The present invention is equally well adapted for improving the range resolution of an FM/CW radar aircraft altimeter.
2. Description of the Background Art
FM/CW radar systems are well known and enjoy many applications. FM/CW radar systems offer many advantages over pulse radar systems, which require interrogation signals having high peak power and a fairly precise time reference to obtain adequate target resolution; oftentimes also requiring elaborate precautions against errors which can arise due to variations in, e.g., pulse width of the interrogating signal in order to maintain acceptable target discrimination. On the contrary, FM/CW radar systems, with their frequency modulated continuous interrogation wave, make possible range resolution of a target simply as a function of frequency and further admit of the advantage of continuous measurement without time reference.
The foregoing theoretical advantages to the contrary notwithstanding, FM/CW radar systems are not without their indigenous problems. Crucial to the accurate resolution of a target is the linearity of the sweep of the frequency of the transmitter signal. As these radar systems rely upon the mixing of the interrogation and reflection signals to achieve a beat or difference frequency indicative of the range of a target, any nonlinearity in the sweep of the transmitter frequency will manifest itself as a false or inaccurate indication. More specifically, where the sweep is linear and when the transmitter signal is then mixed with the receiver signal, the optimal result of a single value beat frequency representing range is achieved. However, where the transmitter signal is not swept linearly, the beat frequency achieved upon mixing will vary as a function of time proportional to the degree of nonlinearity. Hence, a point target will appear extended and the reliability of the system for target discrimination is reduced dramatically.
A principal application of FM/CW systems in the past has been as aircraft altimeters. In that context, some deviation from linearity can be tolerated as it is less important to discriminate points from extended targets. Analysis of the reflected signal for the closest apparent target will normally be reliable, whether it be a point target or an extended target. Accordingly, prior approaches to linearizing the system have focused virtually exclusively on control of the transmitter sweep circuitry itself. For example, most have striven toward the control of the modulation of the continuous wave by the use of feedback loops associated with the transmitter circuitry. Sometimes the approaches are fairly simplistic, other times very complex. In either event, however, the objective is the control on the transmitter side of the system.
Representative of certain prior art systems is that disclosed in U.S. Pat. No. 3,341,849. The patentees there are concerned about the inaccuracies inherent in an FM/CW altimeter due to any nonlinearity in operation of the transmitter system. The principal approach to resolution of the problem is the continuous adjustment of the frequency versus time relationship in the transmitter to insure accurate altitude indications during use and over the range of the instrument. This is achieved, in capsule sum, by monitoring the transmitter, applying a coupled signal as one input to a mixer and a delayed signal as another to generate an error signal proportional to any nonlinearity in the transmitter output; this error signal being utilized to control the transmitter modulation circuitry.
U.S. Pat. No. 4,008,475 discloses a stabilizing and calibrating circuit for FM/CW radar systems. The approach suggested there utilizes continuous feedback to the FM oscillator to account for, and eliminate, drift in the frequency excursions of the FM waveform controlling the transmitter. A control signal is derived by, inter alia, monitoring the transmitter signal and generating a delayed signal representative thereof. This delayed signal is mixed or beat against a signal from the VCO controlling the transmitter in order to obtain a difference signal used for calibration. This difference signal, having been suitably processed, is applied through a loop to control the oscillator.
U.S. Pat. No. 4,106,020 is also concerned with the problem of undesirable changes in FM modulation waveforms for an FM/CW radar. This approach differs conceptually from the foregoing, insofar as the results of variation are compensated rather than the cause of variation (i.e., nonlinearity) being controlled. The disclosed system utilizes a target-simulating delay line and a scaling network to compensate for undesirable deviations in the modulation waveform. In part, this is achieved by applying signals representative of a target to the counting terminal of a counter while a signal representative of a simulated target is coupled to the reset terminal of the counter through a divide-by-N circuit. If the modulation waveform changes, the counter is caused to reset sooner or later by a predetermined amount of time, thereby scaling the target data appropriately, based upon errors in the peak amplitude of the modulation waveform and/or its period. While this patented system discloses a type of correction, it is one simply predicated upon a scaling factor and does not address compensation for nonlinearity in transmitter sweep frequency.
U.S. Pat. No. 3,428,898 is noteworthy as respects the network of the present invention, insofar as the system disclosed in that patent provides means for altering the data sampling rate to account for doppler effects in a satellite communication system. That approach writes data into a delay line storage device at one sample rate frequency while reading it out at a different word rate in order to compensate for doppler effects. Apart from this conceptual feature of variable time sampling to account for temporal data shift, the U.S. Pat. No. 3,428,898 patent offers little practical insight into the linearization of FM/CW radar systems.
U.S. Pat. No. 3,340,529 is interesting in its disclosure of an FM aircraft altimeter designed to reduce so-called "step errors", which arise where target range is in error by an integral number of cycles. Most remarkable about the approach suggested in this reference is that it employs an intentional impression of a nonlinear phase change to overcome this "step error" problem. Thus, as opposed to addressing the problem of nonlinearity in the transmitter frequency sweep, such nonlinearity is intentionally created.
While the aforementioned patented systems involve, to varying degrees, FM/CW ranging systems (or like instruments) and problems associated with certain instabilities thereof, none discloses or suggests the compensation for nonlinearity in transmitter frequency by linearization of the target data signal bearing target range and physical size information. Some approaches involve the attempt to control the sweep of the transmitter frequency to make it linear or as nearly so as can be achieved. To date, no known system has achieved linearity within a tolerance of less than about 0.1-0.5%. However, even as close as that may be to linearity, it remains approximately one order of magnitude too high to afford target discrimination in size and range to identify an extended target from background clutter and/or false targets. Those approaches which do not focus exclusively on efforts to conform transmitter characteristics to make its sweep linear still remain unsatisfactory to achieve the aim of target discrimination. Scaling factors or methods to avoid step error may be suitable for those applications where the system is employed as an aircraft altimeter, since it is typically the closest ground reflection from the earth's surface that is important within such a context; the discrimination of a point from an extended or false target not being an especially crucial distinction in that arena versus the guidance of a missile to a specific target.
Accordingly, the need exists to provide an improved system which begins with as linear a sweep of transmitter frequency as can be economically achieved and which then refines the data signals to compensate for residual nonlinearity which, given the current state of the art and system cost constraints, cannot be eliminated. Furthermore, the need exists to provide such an improved linearization system which yields reliable target information data permitting for range and size resolution on the order of one foot at a target range of up to 2000 feet or more.